Piezoelectric thin-film device process for manufacturing the same, and ink-jet recording head using the same

ABSTRACT

A piezoelectric thin-film device includes: a substrate and a piezoelectric thin film formed on the substrate, wherein a thickness of the piezoelectric thin film is 1 to 10 μm, a crystal grain size of the piezoelectric thin film is 0.05 to 1 μm, and a surface roughness (Rmax) of the piezoelectric thin film is no more than 1 μm.

BACKGROUND OF THE INVENTION

The present invention relates to a piezoelectric thin-film device foruse in ink-jet recording apparatus or the like, a process for itsmanufacture, and an ink-jet recording head using such piezoelectricthin-film device.

Piezoelectric thin films typically made of lead zirconate-titanate(hereinafter sometimes abbreviated as PZT) are formed by various methodsincluding a physical vapor deposition (PVD) technique such assputtering, a chemical vapor deposition (CVD) technique and spin coatingsuch as a sol-gel method, followed by a heat treatment at a hightemperature of 700 to 1000° C. A problem with such piezoelectric thinfilms is that they cannot be made thicker than 1 μm. To solve thislimitation and form thicker piezoelectric films, two approaches areconventionally taken, i.e., increasing the film deposition time orrepeating the film formation several times.

Another approach for increasing the thickness of piezoelectric thinfilms is under review and is based on the use of hydrothermal synthesiswhich permits the intended reaction to proceed in a low-temperature(≦200° C.) environment. According to a recent article entitled"Preparation of PZT crystalline films by hydrothermal synthesis andtheir electrical characteristics" in the preprint for the lectures to beread at the 15th Workshop on Electronic Materials, Japan Institute ofCeramics, the hydrothermal synthesis technique as a method forincreasing the thickness of piezoelectric thin films comprises a seedcrystal forming process in which PZT seed crystals are precipitated onthe surface of a metallic titanium substrate and a crystal growingprocess in which PZT crystals are precipitated and grown on the PZT seedcrystals.

The conventional approaches for manufacturing piezoelectric thin filmsby sputtering, the sol-gel method or the like require a subsequent heattreatment at such high temperatures that they are not suitable forproducing films thicker than 1 μm unless a considerably prolonged timeis spent for film formation. Also cracks will develop even if a film ofdesired thickness is formed.

Thick films can be formed at low temperatures by the hydrothermalsynthesis technique but, on the other hand, the crystal grains producedwill be as large as several micrometers so that neither dense, smoothfilms are formed nor is it possible to achieve fine patterning. In orderfor piezoelectric thin films to be used as piezoelectric devices in anink-jet recording apparatus and the like, the films must be as thick asabout 1 to 10 μm.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstancesand has as an object providing a piezoelectric thin-film device that canbe formed in a sufficient thickness by hydrothermal synthesis, that hasa high electrostriction constant and that permits fine-line patterning.

Another object of the invention is to provide a process formanufacturing the piezoelectric thin-film device.

A further object of the invention is to provide an ink-jet recordinghead using the piezoelectric thin-film device.

The first object of the invention can be attained by a piezoelectricthin-film device that is formed on a substrate and which has apiezoelectric thin film having a thickness of 1 to 10 μm, a crystalgrain size of 0.05 to 1 μm and a surface roughness (Rmax) of no morethan 1 μm.

If the crystal grain size of the piezoelectric thin film is at least0.05 μm and more, the required piezoelectricity can be exhibited. If thecrystal grain size of the piezoelectric film is no more than 1 μm, thepiezoelectric thin-film device permits fine-line patterning. Thesenumerical conditions can be satisfied by a structure that is created bythe growth of a piezoelectric thin film on nuclei composed of the fineseed crystals of a piezoelectric thin film.

If the surface roughness (Rmax) of the piezoelectric thin film is notmore than 1 μm, the latter can be fully covered with an upper electrode.

The lower electrode which makes a pair with the upper electrode ispreferably made of platinum. The piezoelectric thin film is oriented ineither the (100) or (111) plane. The piezoelectric thin film preferablyhas a crystal grain size of 0.1 to 0.5 μm. The upper electrodepreferably has a thickness 0.5 to 2 times as great as the surfaceroughness (Rmax) of the piezoelectric thin film.

If the piezoelectric thin film is at least 1 μm thick, it provides thepiezoelectricity required, for example, by the ink-jet recording head.If the piezoelectric thin film is not thicker than 10 μm, the desiredpiezoelectric thin-film devices can be fabricated at high density.Preferably, the thickness of the piezoelectric thin film is 2 to 5 μm,with 3 μm being more preferred.

The present invention also provides a piezoelectric thin-film devicethat is formed on a substrate and which has a piezoelectric thin film ofsuch a structure that crystals have been grown on nuclei composed offine seed crystals. In a preferred embodiment, the piezoelectric thinfilm has such a structure that crystals have been grown on nucleicomposed of PZT seed crystals by hydrothermal synthesis. The PZT seedcrystals are produced by either physical vapor deposition (PVD) orchemical vapor deposition (CVD) or spin coating. As already mentioned,the seed crystals desirably have a grain size of 0.05 to 1 μm.

The second object of the invention can be attained by a process formanufacturing a piezoelectric thin-film device which includes the stepof forming the seed crystals of a piezoelectric thin film on a substratewith a lower electrode by either physical vapor deposition (PVD) orchemical vapor deposition (CVD) or spin coating and then causing thecrystals of a piezoelectric thin film to grow on the seed crystals of apiezoelectric thin film by hydrothermal synthesis. The seed crystals ofa piezoelectric thin film are specifically formed by either a sol-gelmethod or a sputtering technique. The seed crystals of a piezoelectricthin film are desirably oriented in the (100) plane if they are formedby a sol-gel method and, in the case where they are formed bysputtering, the orientation is desirably in the (111) plane.

The third object of the invention is attained by an ink-jet recordinghead comprising a substrate having ink chambers formed therein, avibrating diaphragm that closes the ink chambers at one end and whichhas piezoelectric thin-film devices of a flexural vibrating mode fixedon a surface, and a nozzle plate that closes the ink chambers at theother end and which has ink-ejecting nozzle holes formed therein,wherein each of the piezoelectric thin-film devices has a piezoelectricthin film created by the growth of crystals through hydrothermalsynthesis on the seed crystals of the piezoelectric thin film that havebeen formed by either physical vapor deposition (PVD) or chemical vapordeposition (CVD) or spin coating and which have a crystal grain size of0.05 to 1 μm. In a preferred embodiment, each of the piezoelectricthin-film devices has both an upper and a lower electrode forelectrically charging the piezoelectric thin film and also has means ofelectrically charging the upper electrode such that it has a positivepotential relative to the lower electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of the piezoelectric thin-filmdevice according to a first and second embodiments of the presentinvention;

FIG. 2a is a simplified perspective view of the ink-jet recording headaccording to a third embodiment of the invention;

FIG. 2b is an enlarged section taken on line A--A of FIG. 2a;

FIGS. 3a to 3c show in section three major steps of the process formanufacturing the ink-jet recording head according to the thirdembodiment of the invention;

FIG. 4 is a timing chart showing the waveform of a pulsed voltage to beapplied to the recording head shown in FIG. 2a;

FIG. 5 is a characteristic diagram showing the relationship between thevoltage applied to a PZT film and a d constant for its displacement; and

FIG. 6 is a table showing how the rate of temperature elevation in thepreparation of PZT seed crystals is related to the grain size of theseed crystals, the surface roughness of the PZT film grown byhydrothermal synthesis and its electrostriction constant (d₃₁).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedbelow.

First Embodiment

<Formation of Seed Crystals>

Lead acetate (0.1 mole) was dissolved in 20 mL of acetic acid and thesolution was refluxed for 30 min. After reversion to room temperature,zirconium tetrabutoxide (0.052 moles) and titanium tetraisopropoxide(0.048 moles) were dissolved. One mole of water and a small amount ofdiethylene glycol were added dropwise thereto, and the mixture wasstirred thoroughly to effect hydrolysis. Following dilution with2-ethoxyethanol, hudroxypropyl chamberulose was added in an amount of 5wt % of the value calculated for a complex metal oxide and the mixturewas stirred thoroughly to form a uniform sol.

FIG. 1 shows a schematic cross section of a piezoelectric thin-filmdevice. The sequence of steps in the manufacture of the piezoelectricthin-film device are described below. First, a single-crystal siliconsubstrate 101 is overlaid with a silicon dioxide film 201 as aninsulator film by thermal oxidation. Subsequently, a Pt lower electrode104 is formed on the silicon dioxide film 201 by sputtering and thenspin-coated with a sol that has been prepared as already describedabove, and then calcined at 350° C.

The above procedure enabled an amorphous porous gel thin film to form ina thickness of 0.4 μm without any cracking. The lower electrode wasformed of Pt since it would not deteriorate upon intense heat treatment.

In the next step, the calcined substrate was heated in a diffusionfurnace at 550° C. and held at that temperature for 1 h (hour), wherebypreannealing was effected to form a thin dense film in a thickness of0.3 μm. By X-ray analysis, peaks for crystals of a perovskite type weredetected. The film was subjected to reflective FT-IR (Fourier transforminfrared absorption spectral analysis) and no absorption was to haveoccurred due to hydroxyl groups at about 3400 cm⁻¹.

Subsequently, annealing was performed in a diffusion furnace under acirculating oxygen flow by heating at 700° C. for 1 h. Examination ofthe resulting film with a metallurgical microscope showed that it had onits surface those PZT seed crystals 105 (the seed crystals of apiezoelectric thin film) which had been formed in a thickness of 0.3 μm,with the crystal grains having grown to a size of 0.5 μm.

By X-ray analysis of the PZT seed crystal layer 105, there were detectedsharp and intense peaks peculiar to perovskite-type crystals. In thisconnection, if the PZT seed crystals obtained by the sol-gel method areto be used as seed crystals in hydrothermal synthesis, the crystals aredesirably oriented in the (100) plane in order to ensure that theintended piezoelectricity would be realized.

<Crystal Growth>

In the next step, the PZT seed crystals 105 formed by the sol-gel methoddescribed above were subjected to hydrothermal reaction, thereby causinga PZT film (layer) 106 to grow on the seed crystals. The reactionsolution used in the hydrothermal reaction was prepared by mixingaqueous solutions of lead nitrate Pb(NO₃)₂, zirconium oxychlorideZrOCl₂, titanium chloride TiCl₄ and potassium hydroxide KOH.

The other side of the silicon substrate 101 on which the PZT seedcrystals 105 were formed was coated with a fluoroplastic resin andwetted with the reaction solution (see above) at 150° C. to perform ahydrothermal treatment for 12 h, whereupon the PZT film 106 grew to athickness of 3 μm.

Subsequently, an aluminum electrode was deposited on the PZT film 106 byevaporation and the PZT film 106 was found to have salient piezoelectriccharacteristics as evidenced by a specific dielectric constant of 1200and a electrostriction constant of 90 pC/N.

The PZT film 106 was also dissolved in aqua regia and the molar ratio ofits components (Pb, Zr and Ti) was measured by ICP-AES(inductively-coupled plasma-atomic emission spectroscopy); the resultwas Pb:Zr:Ti=1:0.52:0.48, which was identical to the composition of theinitial feed.

The crystal grains in the PZT film 106 formed by hydrothermal synthesiswere of substantially the same size as the PZT seed crystals 105 and thePZT film 106 had a smooth surface with Rmax being 0.4 μm. Thus, by meansof the hydrothermal synthesis technique, the PZT film 106 could beformed in such a way that the crystal grain size and the surfaceroughness (Rmax) were substantially the same as in the layer of PZT seedcrystals 105.

In other words, the smaller the size of the PZT seed crystals 105, thedenser and the smoother the PZT film 106 that is formed. The requiredpiezoelectricity can be ensured if the size (=grain size=diameter) ofthe PZT seed crystals 105 is at least 0.05 μm and more.

As long as the size of the PZT seed crystals 105 is not more than 1 μm,a good surface smoothness is provided to ensure that the upper electrodecovers the entire surface of the PZT film 106. Preferably, the grainsize of the PZT seed crystals 105 is adjusted to lie between 0.1 and 0.5μm, which achieves a further improvement in the characteristicsdescribed above. The grain size of the PZT seed crystals 105 to beobtained by the sol-gel method can be controlled by adjusting thesintering speed and time. The PZT seed crystals 105 will serve thepurpose if the film formed of those crystals is thick enough to coverthe entire surface of the substrate.

We now describe in detail the method of forming PZT seed crystals ofvarying thickness by the sol-gel technique. In the sol-gel method, aheat treatment is performed at 700° C. for 1 h to crystallize PZT andthe crystal grain size can be varied by adjusting the rate oftemperature elevation. FIG. 6 is a table showing how the rate oftemperature elevation is related to the grain size of seed crystals, thesurface roughness of the film formed by growth in the hydrothermalsynthesis method and its electrostriction constant (d₃₁). As the grainsize increases, the electrostriction constant increases to providebetter piezoelectric characteristic. However, in order to achievefine-line pattering, the grain size of the PZT seed crystals ispreferably not more than 1 μm. As already noted, in the piezoelectricthin-film device of the invention the piezoelectric thin film has athickness of 1 to 10 μm, a crystal grain size of 0.05 to 1 μm and asurface roughness (Rmax) of not more than 1 μm. In order to obtain theseconditions, the PZT seed crystals are preferably heated at a rate of 3to 53° C./min. If the grain size of these seed crystals is in the rageof 0.1 to 0.5 μm, the balance between the two needs, one to ensure therequired piezoelectric characteristics and the other to permit fine-linepatterning, can be attained at a higher level.

In the first case described above, the PZT film 106 is of a purelytwo-component system. This is not the sole case of the invention and inorder to provide better piezoelectric characteristics, the PZT film 106may be formed of a three-component system such as one consisting oflead-magnesium niobate, lead zirconate and lead titanate; if desired,various additives may be added for various purposes, for example, ironmay be added to improve the withstand voltage and chromium may be addedto reduce the change with aging. In these modified cases, thecomposition of the sol solution and that of the hydrothermal reactionsolution should of course be altered in accordance with the compositionof the PZT film to be formed.

Second Embodiment

We now describe the second embodiment of the invention. In this secondembodiment, the piezoelectric thin-film device shown in FIG. 1 ismanufactured by the process described below.

First, a single-crystal silicon substrate 101 was overlaid with asilicon dioxide film 201 as an insulator film by thermal oxidation. Aplatinum (Pt) lower electrode 104 was then formed over the silicondioxide film 201 by sputtering.

Then, using a target having a PbZrO₃ :PbTiO₃ molar ratio of 52:48, anamorphous PZT film was deposited by sputtering in a thickness of 0.3 μm.Thereafter, the amorphous PZT film was crystallized by a heat treatmentfor 1 h at 750° C. in an oxygen atmosphere, whereupon the PZT film wastransformed to a layer of PZT seed crystals 105 such that the size ofthe crystal grains observed on the film surface with a metallurgicalmicroscope was 0.4 μm and that those seed crystals would exhibitpiezoelectricity.

By X-ray analysis of the PZT seed crystals 105, there were detectedsharp and intense peaks derived from the perovskite-type crystals. Itshould be mentioned here that if the PZT seed crystals 105 obtained bysputtering are to be used as seed crystals in hydrothermal synthesis,the crystals are desirably oriented in the (111) plane in order toensure that the intended piezoelectric characteristics can be realized.

As in the first case, the PZT seed crystals 105 formed by sputteringwere subjected to a hydrothermal treatment. The reaction solution wasprepared by mixing aqueous solutions of lead nitrate Pb(NO₃)₂ zirconiumoxychloride ZrOCl₂, titanium chloride TiCl₄ and potassium hydroxide KOH.The hydrothermal treatment was performed for 12 h, for which time thesingle-crystal silicon substrate 101 having the PZT seed crystals 105formed on one surface and which was coated with a fluoroplastic resin onthe opposite side was immersed in the reaction solution as the latterwas held at 150° C. As the result of the hydrothermal treatment, a PZTfilm 106 was formed in a thickness of 3 μm.

Subsequently, an aluminum electrode was deposited on the PZT film 106 byevaporation and the PZT film was found to have salient piezoelectriccharacteristics as evidenced by a specific dielectric constant of 1100and a electrostriction constant of 85 pC/N. The thin PZT was alsodissolved in aqua regia and the molar ratio of its components (Pb, Zrand Ti) was measured by ICP-AES; the result was Pb:Zr:Ti=1:0.52:0.48,which was identical to the composition of the initial feed.

The crystal grains in the PZT film 106 formed by hydrothermal synthesiswere 0.4 μm in size which was equal to that of the PZT seed crystals 105and the PZT film 106 had a smooth surface with Rmax being 0.4 μm.

In the second embodiment, the PZT seed crystals 105 prepared bysputtering were employed; needless to say, similar results will beobtained even if the PZT seed crystals are formed by chemical vapordeposition (CVD) techniques.

Third Embodiment

The third mode of embodiment of the invention will now be described withreference to FIG. 2a which is a simplified perspective view of anink-jet recording head and FIG. 2b is an enlarged section taken on lineA-A' of FIG. 2a.

The ink-jet recording head shown in FIGS. 2a and 2b comprises asingle-crystal silicon substrate 101, ink chambers 102 formed on thesingle-crystal silicon substrate 101, a silicon dioxide film 201 formedover the ink chambers 102, piezoelectric devices that are formed on thesilicon dioxide film 201 and which each has a Pt lower electrode (layer)104, a layer (film) of PZT seed crystals 105, a PZT film 106 and anupper electrode (layer) 107, and a nozzle plate 108 that is joined tothe lower side of the single-crystal silicon substrate 101 and whichhave nozzles 109 formed therein. The ink chambers 102 are arranged onthe same pitch as the nozzles 109.

The ink-jet recording head operates as follows. When a voltage isapplied between the Pt lower electrode 104 and the upper electrode 107,the piezoelectric device comprising the Pt lower electrode 104, the PZTseed crystal layer 105, the PZT film 106 and the upper electrode 107, aswell as the silicon dioxide film 201 are deformed. As a result, thecapacity of each ink chamber 102 is reduced, to pressurize its interior,such that the ink filling the chamber 102 is ejected out of the chambervia the associated nozzle 109.

Each of the ink chambers 102 has a length of 100 μm in the direction inwhich they are arranged side by side (to the right and left of FIG. 2b)and has a length of 4 mm in the direction in which they extendlongitudinally (normal to the paper); each of the PZT seed crystallayers 105 has a length of 80 μm in the direction in which they arearranged side by side (to the right and left of FIG. 2b).

The ink chambers 102 are arranged side by side at a pitch of 141 μm toprovide a resolution of 180 dpi (dots per inch). In the illustratedcase, the PZT seed crystal layer 105 and the PZT film 106 are providedonly in the area above the ink chamber 102 whereas they are not providedin the area that corresponds to the gap between adjacent ink chambers102; this arrangement ensures that all ink chambers 102 will bedisplaced by the same amount even if a small voltage is applied.

<Manufacture of the Ink-Jet Recording Head>

We now describe the process for the manufacture of the ink-jet recordinghead with reference to FIGS. 3a to 3c. FIGS. 3a to 3c show in crosssection the sequence of steps for the manufacture of the ink-jetrecording head. In each of the cross sections shown in these figures,the ink chambers 102 are supposed to extend longitudinally in thedirection normal to the paper.

In the first step of the process shown in FIG. 3a, a single-crystalsilicon substrate 101 having the plane orientation (110) and which has athickness of 220 μm is thermally oxidized at 1200° C. by a wet method toform silicon dioxide films 201 and 202 simultaneously on opposite sidesof the single-crystal silicon substrate 101.

In the next step, the silicon dioxide film 201 is overlaid with a Ptlower electrode 104, PZT seed crystal layers 105 (formed by the sol-gelmethod), PZT films 106 (formed by hydrothermal synthesis) and upperelectrodes 107. The PZT seed crystal layers 105 and the PZT films 106may be formed by the sol-gel and hydrothermal synthesis methods,respectively, as in the first case, except that the PZT seed crystallayers had a film thickness of 0.3 μm whereas the PZT films had athickness of 3 μm.

In practice, intermediate layers were formed between the silicon dioxidefilm 201 and the Pt lower electrode 104 in order to provide betteradhesion; as such intermediate layers, titanium, titanium dioxide andtitanium were deposited in that order in respective thicknesses of 250,200 and 50 angstroms. The three intermediate layers (Ti, 250 Å; TiO₂,200 Å; Ti, 50 Å) and the Pt lower electrode 104 were successively formedby DC sputtering, except that the titanium dioxide was deposited byreactive sputtering in a 10% O₂ atmosphere.

In the next step, upper electrodes 107 were formed on the PZT film 106by depositing platinum in a film thickness of 3000 Å through DCsputtering. By thusly ensuring that the film thickness of the upperelectrode 107 is 0.5 to 2 times as great as the surface roughness(Rmax=0.4 μm) of the PZT film 106, all asperities in the surface of thePZT film 106 can be covered, thereby providing satisfactory coveragecharacteristics. Since the film thickness of the upper electrode 107 isnot more than twice the surface roughness of the PZT film, the desireddisplacement of the piezoelectric devices can be assured (the upperelectrode 107 will not prevent the piezoelectric devices from beingdisplaced).

Subsequently, a photoresist was applied to the silicon dioxide film 202and openings were made in selected positions of the photoresist. Withthe thus worked photoresist being used as a mask, the dioxide siliconfilm 202 was patterned with hydrofluoric acid and an aqueous solution ofammonium fluoride. Openings 203 were thus formed in the silicon dioxidefilm 202 and the direction in which they extended longitudinally (i.e.,normal to the paper) was set to be parallel to the (112) plane of thesingle-crystal silicon substrate 101.

The process then goes to the step shown in FIG. 3b and the upperelectrodes 107, PZT films 106 and PZT seed crystal layers 105 are etchedin a pattern by ion milling. The patterning was such that the upperelectrodes 107, PZT films 106 and PZT seed crystal layers 105 would beleft intact in the areas corresponding to the openings 203. Theprecision in patterning was satisfactory since the grain sizes of thePZT seed crystals 105 and of the crystals in the PZT film 106 were assmall as 0.5 μm.

In the next step shown in FIG. 3c, the surface of the single-crystalsilicon substrate 101 on which the piezoelectric devices have beenformed is protected with a jig and the substrate is immersed in anaqueous solution of potassium hydroxide at 80° C. so that it is etchedanisotropically with the silicon dioxide film 202 being used as a maskto form ink chambers 102.

Since the single-crystal silicon substrate 101 is oriented in the (110)plane whereas the individual openings 203 extend longitudinally in adirection parallel to the (112) plane, the two side walls of each inkchamber 102 which extend along its length can be oriented to be parallelto the (111) plane.

If an aqueous solution of potassium hydroxide is used as the liquidetchant, the etch rate ratio for the single-crystal silicon in the (110)and (111) planes is about 300:1; hence, the degree of side etching whichwill occur during the formation of grooves equal in depth to thethickness (220 μm) of the single-crystal silicon substrate 101 can bereduced to about 1 μm, thereby achieving highly precise formation of inkchambers 102.

Subsequently, with the piezoelectric surface of the single-crystalsilicon substrate 101 being protected with the same tool, the silicondioxide film 202 is etched away with hydrofluoric acid and an aqueoussolution of ammonium fluoride. Thereafter, the other side of thesubstrate on which the ink chambers 102 are open is subjected to desiredprocessing such as joining with a nozzle plate, whereupon a completeink-jet recording head is produced.

The foregoing description of the third embodiment of the inventionconcerns the case of continuously patterning the PZT seed crystal layers105, PZT films 106 and upper electrodes 107 by ion milling; it should,however, be noted that this is not the sole case of the invention andmay be replaced by another approach such as this: the layer of PZT seedcrystals 105 is first patterned by ion milling, then the PZT film 106 isprecipitated selectively and finally the upper electrode 107 isdeposited and patterned.

(An Operation of the Ink-Jet Recording Head)

The ink-jet recording head shown in FIG. 2 may be operated in thefollowing specific manner. First, the findings of the present inventionwill be described. During their deposition, the PZT films 105 and 106have been polarized spontaneously in a specified direction. Hence, anelectric field of 20 V/μm is usually applied to these PZT films suchthat the polarization is re-oriented in a desired direction (thisprocess is commonly referred to as "poling"). However, particularly inthe case of PZT films prepared by the hydrothermal synthesis technique,even this poling step is incapable of reorienting the spontaneouspolarization in a specified direction.

If, in this situation, the polarities of the upper electrode 107 and thelower electrode are not controlled in any way, a problem will arise inthat depending on the direction of the applied electric field, it maynot be possible to ensure that the PZT films are displaced bysufficiently large amounts (in terms of the width of their shift towardthe ink chambers 102). According to the result of the intensive studymade by the present inventors, it was found that if the potential of theupper electrode is made higher than that of the lower electrode,preferably by applying the ground potential to the lower electrode whileapplying a positive potential to the lower electrode, the PZT films aredisplaced by a little more than about twice the displacement whichoccurs in the opposite case (i.e., a positive potential is applied tothe lower electrode and the ground potential to the upper electrode).

Stated specifically, the PZT films were each displaced by 150 nm when apulsed potential (20 V/μm) of the waveform shown in FIG. 4 was appliedto the upper electrode. In contrast, the displacement of the PZT filmswas no more than 70 nm when the same potential was applied to the lowerelectrode with the upper electrode being grounded.

FIG. 5 is a characteristic diagram showing how the polarization of PZTfilms having a total thickness of 1 μm is related to a d constant (d₃₁in pC/N) which is in proportion to the displacement of PZT films. As isclear from FIG. 5, electrically charging the upper electrode to thepositive side relative to the lower electrode causes the PZT films to bedisplaced by a greater amount than in the opposite case.

Except for the application of voltage in the manner just describedabove, known procedures of field application may be adopted.

It should be noted that if the upper electrode is to be electricallycharged to the positive side relative to the lower electrode, theelectrolytic corrosion of the lower electrode can be prevented by makingthe upper electrode of either one of Al, Ni and Ti which have lowerredox potentials than Pt (which is exclusively used in the lowerelectrode in high-temperature processes).

The foregoing description of the invention concerns just one example andit should be understood that the proportions of the constituents of theseed crystal for the piezoelectric thin film, the kind of the feedmaterials to be used and other features of the invention are by no meanslimited to the illustrated case.

(Advantages of the Invention)

As described on the foregoing pages, seed crystals prepared by thesol-gel method or the sputtering process which permit the crystal grainsize and orientation to be controlled are employed in forming apiezoelectric thin film by the hydrothermal synthesis technique and thisenables dense, smooth and highly piezoelectric thin films to be formedin thicknesses of 1 μm and more. As a result, piezoelectric thin-filmdevices that have high electrostriction constant and which permitfine-line patterning can be manufactured in high yield. The ease withwhich such piezoelectric thin films can be manufactured offers the addedadvantage of realizing low-cost manufacture of ink-jet recording headsthat use high-performance piezoelectric thin-film devices that need befabricated in very small feature sizes.

What is claimed is:
 1. A piezoelectric thin-film device comprising:asubstrate; and a piezoelectric thin film formed on the substrate,wherein a thickness of the piezoelectric thin film is 1 to 10 μm, agrain size of crystals of the piezoelectric thin film is 0.05 to 1 μm,and a surface roughness (Rmax) of the piezoelectric thin film is no morethan 1 μm.
 2. The piezoelectric thin-film device according to claim 1,further comprising:upper and lower electrode portions provided onrespective upper and lower surfaces of the piezoelectric thin film forapplying an electric field thereto, wherein the lower electrode portionis made of platinum.
 3. The piezoelectric thin-film device according toclaim 1, wherein the piezoelectric thin film has a crystal structureoriented in the (100) or (111) plane.
 4. The piezoelectric thin-filmdevice according to claim 1, wherein the crystals of the piezoelectricthin film have a crystal size of 0.1 to 0.5 μm.
 5. The piezoelectricthin-film device according to claim 2, wherein the upper electrodeportion has a thickness 0.5 to 2 times as great as the surface roughness(Rmax) of the piezoelectric thin film.
 6. A piezoelectric thin-filmdevice comprising:a substrate; and a piezoelectric thin film formed onthe substrate; wherein the piezoelectric thin film has a structurewhereby crystals have grown on nuclei composed of fine seed crystals;and wherein a thickness of the piezoelectric thin film is 1 to 10 μm, agrain size of crystals of the piezoelectric thin film is 0.05 to 1 μm,and a surface roughness (Rmax) of the piezoelectric thin film is no morethan 1 μm.
 7. The piezoelectric thin-film device according to claim 6,wherein the crystals of the structure of the piezoelectric thin filmhave grown on nuclei composed of PZT seed crystals by hydrothermalsynthesis.
 8. The piezoelectric thin-film device according to claim 7,wherein the PZT seed crystals are produced by either physical vapordeposition (PVD) or chemical vapor deposition (CVD) or spin coating. 9.An ink-jet recording head comprising:a substrate having ink chambersformed therein; a vibrating diaphragm closing the ink chambers at oneend and having a piezoelectric thin-film device of a flexuraloscillating mode fixed on a surface of the vibrating diaphragm; and anozzle plate closing the ink chambers at the other end and havingink-ejecting nozzle holes formed therein; wherein the piezoelectricthin-film device has a piezoelectric thin film created by the growth ofcrystals through hydrothermal synthesis on seed crystals of thepiezolelectric thin film that have been formed by either physical vapordeposition (PVD) or chemical vapor deposition (CVD) or spin coating; andwherein a thickness of the piezoelectric thin film is 1 to 10 μm, agrain size of crvstals of the piezoelectric thin film is 0.05 to 1 μm,and a surface roughness (Rmax) of the piezoelectric thin film is no morethan 1 μm.
 10. The ink-jet recording head according to claim 9, whereinthe piezoelectric thin-film device has both an upper and a lowerelectrode for electrically charging the piezoelectric thin film and alsohas means for electrically charging the upper electrode such that it hasa positive potential relative to the lower electrode.